Conventionally, a two-spool turbojet comprises an engine nacelle housing a high pressure spool comprising a high pressure compressor and a high pressure turbine, and a low pressure spool comprising a low pressure compressor and a low pressure turbine and driving the fan.
The function of such a turbojet is generally not only to provide the airplane with thrust, but also to provide the airplane with electricity for various purposes such as powering computers, cabin lighting, etc.
The present trend is to increase the amount of mechanical power that is taken off and converted into electrical power, given the increasing role played by electrical means, which have the reputation of being flexible in use on board an airplane.
An electricity generator is used for transforming a fraction of the mechanical power delivered by the turbojet into electricity.
The generator is not usually situated in the turbojet. A secondary shaft is connected via gearing to one of the shafts of the turbojet, thereby setting it into rotation when the engine is operating, and thus transmitting thereto the mechanical power needed to make the electricity generator operate. The generator may be placed close to the end of the shaft, outside the turbojet.
In another known configuration, described in PCT international application WO 2007/036202, the electricity generator may also be arranged inside the turbojet, and more precisely inside the high pressure spool. Advantageously, that generator operates reversibly, so that it also acts as a starter.
When starting a turbojet, the high pressure spool is the first to be put into rotation using an electricity generator operated as an electric motor. The high pressure compressor then feeds the combustion chamber with air under pressure, thereby enabling combustion to be established, and enabling the high pressure stage of the turbojet to start. The low pressure turbine is then driven in rotation by the exhaust stream of primary air, thereby driving the low pressure spool and the fan. Once the turbojet has started, the power supply to the electric starter is switched off so it ceases to act as a motor driving the high pressure compressor.
Arranging the electricity generator in the high pressure spool has as its main advantage the fact that the generator can be used both as an electric motor for starting the turbojet, as described above, and also as an electricity generator for delivering electricity to the remainder of the airplane.
Nevertheless, that configuration presents various drawbacks. The most noticeable is the poor overall efficiency with which mechanical power is converted into electrical power when the electricity generator is disposed in that way. The extra consumption of fuel needed to provide given electrical power is greater when the electricity generator takes power from the high pressure spool than when it takes power from the low pressure spool.
Furthermore, arranging the electricity generator in the high pressure spool is difficult. The spool constitutes a region of small diameter that includes numerous pipes. The generator is thus difficult to put into place and to maintain.
Furthermore, since the vanes of the high pressure compressor are generally variable-pitch vanes, it is difficult to reduce the distance or airgap between the rotor and the stator. As a result, the electricity generator made in that way usually presents relatively low efficiency.
Finally, installing an electricity generator that draws its power from the high pressure spool can raise a problem of turbojet operability. If too much mechanical power is taken, then there is a negative effect on the operation of the high pressure spool that can lead to pumping of the high pressure compressor, in particular when the engine is operating at slow speed.
Another known way of taking mechanical power from a turbojet in order to produce electricity is described in PCT international application WO 2005/073519.
In order to generate electricity, that document proposes adding an additional turbine that is dedicated to producing electricity and that is located downstream from the fan in the secondary stream of a turbojet, said turbine rotating independently of the spools of the turbojet.
Nevertheless, adding that turbine makes the structure of the turbojet much more complex and increases its weight and its volume. In addition, the efficiency with which mechanical power is converted into electrical power is relatively low.